a. Field of the Invention
This invention relates to processes for the preparation of pyridine-tetracarboxylic acids and to their conversion to the corresponding pyridines, and to intermediates used therein.
B. Description of the Prior Art
A wide variety of synthetic methods for preparing pyridines from acyclic compounds are presented at pages 272 thru 589 of "Pyridine and Its Derivatives, Part I", Klingsberg, Editor, (Interscience Publishers, Inc., New York, 1960). At pages 498-533 under the heading "2-1-2 Condensation", there are presented many illustrations of the "reaction of an aldehyde with ammonia and the second aldehyde or ketone . . . (to produce) a pyridine or its dihydro derivative" (page 498), as given in Tables II-121 thru II-125. In the first complete paragraph of page 500, it is taught:
"Simple aldehydes and ketones without activating groups require mildly forcing conditions and produce the fully aromatic pyridines; accordingly, acetophenone, benzaldehyde, and ammonia in refluxing acetic acid gave a 68% yield of 2,4,6,-triphenylpyridine and a 25% yield of .beta.-phenylpropionophenone." PA1 "The condensation of ammonia and an aldehyde with an active methylenic ketone (The Hantzsch Synthesis) such as ethyl acetoacetate requires only mild conditions and produces a dihydropyridine." There follows illustrations of the preparation of various lower-alkyl 1,4-dihydro-3,5-pyridinedicarboxylates; however, there is no teaching of the synthesis of any lower-alkyl 1,4-dihydro-2,3,5,6-pyridinetetracarboxylates.
The first sentence of the next paragraph (page 500) reads: